The present invention relates to a straddle-type all-terrain vehicle and, more particularly, to a straddle-type all-terrain vehicle with a limited-slip differential.
In recent years, there has been a growing demand for four-wheel drive straddle-type all-terrain vehicles as opposed to the simpler two-wheel drive variety. This has been largely due to the desire to drive these vehicles in increasingly rough off-road conditions, which require improved traction and improved capability to transmit engine torque to the wheels.
Presently on the market are two types of four-wheel drive straddle-type all-terrain vehicles (ATV""s). The first type employs a regressive limited-slip differential at the front and a fully locked axle at the rear. The front differential is either of the speed-sensing or torque-sensing variety. Examples of regressive differentials are the Sure-Trac(trademark) and the Detroit Locker(trademark). These types of differentials will be discussed in greater detail later. The second type of drive train found on the market today uses an over-running clutch in the front and a fully locked axle at the rear.
The regressive limited-slip differential permits engine torque to be transferred equally to the front drive axles provided that the tires have equal traction. A regressive limited-slip differential transfers torque from the slipping wheel to the wheel with traction as a function of the differential wheel speed. However, the transfer of torque diminishes as the wheel speed differential increases, eventually reaching a plateau beyond which no more torque can be transferred (hence the name xe2x80x9cregressivexe2x80x9d).
The over-running clutch functions according to the difference in wheel speed between the front and rear wheels. When the front and rear axles are turning at the same speed, the vehicle is powering only the rear wheels (2xc3x974 mode). However, if the difference in axle speed increases beyond a threshold value, the over-running clutch couples the front drive axle to the rear drive axle (4xc3x974 mode). Since the vehicle runs mainly in 2xc3x974 mode, steering is easy. However, when wheel slippage occurs and power is transferred also to the front wheels (4xc3x974 mode), vehicle performance becomes less than optimal.
Most current 4xc3x974 ATV""s are equipped with a solid rear drive axle. This provides optimal traction because torque is delivered to both rear drive wheels all the time irrespective of the ground conditions. Due to the short wheelbase, it is not necessary to employ a rear differential because the rear inner wheel tends to lift during cornering, allowing natural, non-detrimental wheel slippage. However, when hauling heavy loads on the rear of the ATV, the downward load precludes the natural wheel lift during cornering, thus making it necessary to employ an open differential at the rear or else suffer the consequences of inner rear tire drag. Some ATV""s have a manually selectable locking rear axle that allows the user to select either an open rear differential or a closed, locked axle depending on the traction requirements.
In order to fully appreciate the present invention, it is preferable to understand the structure and functioning of a differential.
When cornering, the outer wheels of an straddle-type all-terrain vehicle travel a greater distance (and thus rotate at a greater angular velocity) than the inner wheels. For a non-driven axle, the left and right wheels are free to rotate independently of each other and thus each wheel follows at its own angular velocity. However, for wheels that are coupled to a driven axle, it is necessary to employ a differential (or xe2x80x9cdiffxe2x80x9d) to allow the outer wheel to turn faster than the inner wheel while still transmitting torque to both wheels.
In the absence of a differential, the inner wheel during cornering will drag because most of the weight is on the outer wheel and thus the inner wheel is forced to rotate at the same angular velocity as the outer wheel even though the inner wheel has a shorter distance to travel. The partial dragging of the inner wheel is detrimental to tire wear, comfort and control and furthermore promotes understeer (the tendency of the vehicle to continue in a straight line). Hence the necessity for a differential capable of permitting the inner wheel to rotate at a lesser angular velocity than the outer wheel during cornering while still distributing torque to both the inner and outer wheel.
Engine torque is transmitted through the clutch and transmission gears to a drive pinion mounted at the end of the drive shaft. This drive pinion is typically a bevel gear (either straight or spiral) meshed with a perpendicular (also beveled) ring gear. A differential case (or housing) is fixed to the ring gear so that the case revolves with the ring gear about an axis defined by the drive axle. Inside the differential case is typically a pinion shaft mounted to the differential case and rotatable therewith in the plane perpendicular to the axis defined by the drive axle. Mounted on the pinion shaft are two differential beveled pinion gears which revolve with the ring gear and case but which are also free to rotate about the pinion shaft axis. The differential pinion gears are meshed with a pair of beveled side gears which are each splined to a drive axle. This arrangement is termed an xe2x80x9copen differentialxe2x80x9d. These devices are also colloquially known as xe2x80x9cforce balancersxe2x80x9d since they distribute an equal amount of torque to each drive axle irrespective of the rotational speed of each axle. In operation, the rotation of the drive pinion causes the ring gear and the differential case to revolve about the axis defined by the drive axles. Since the differential pinion gears are mounted to the pinion shaft, they revolve in a planetary fashion around the aforementioned axis and, in so doing, drive the meshed side gears (thereby driving each of the drive axles).
When traveling in a straight line (and not exceeding the traction limit of the tires), the differential pinions and side gears move with the case. There is no relative movement between the teeth of the differential pinions and those of the side gears.
When cornering, the differential pinions still revolve with the ring gear and differential case. Since the outer wheel must now rotate faster than the inner wheel, the differential pinions must now also rotate about their shaft axis. The rotation of the differential pinions compensates for the differential rotation of the outer and inner axles. Nevertheless, the rotation of the differential pinions does not affect the transfer of torque to each of the side gears. Thus, the differential pinions continue to exert a torque on each of the side gears, which in turn, transmit torque to the drive wheels.
An open differential can compensate for any variation in axle speed up to the traction limit of the wheels. In other words, when one wheel begins to slip or spin, the differential case continues to revolve (assuming the torque being transmitted from the drive pinion to the ring gear remains constant, i.e., assuming constant engine torque). As the differential case revolves, the differential pinions continue to revolve around the side gears. However, since the side gear coupled to the free-spinning wheel presents a much smaller rotational inertia, the differential pinions will transfer all of the available torque to the side gear coupled to the free-spinning wheel. Thus, no torque is transferred to the wheel with the greater traction resulting in a complete loss of traction. In the industry jargon, it is said that the differential pinions xe2x80x9cwalkxe2x80x9d around the axle connected to the wheel with the greater traction. In other words, the differential pinions xe2x80x9ctake the path of least resistancexe2x80x9d by driving the side gear splined to the free-spinning wheel instead of driving the wheel with traction.
Thus, although the open differential distributes torque equally to both wheels when cornering, the open differential is susceptible to wheelspin (and thus loss of traction and control) whenever the propulsive force generated by the wheel (the dot product of wheel torque and tire radius) exceeds the traction limit.
To overcome the shortcomings associated with open differentials, limited-slip differentials were introduced which, as their name implies, limits slippage of the tire with lesser traction and transfers torque to the tire with greater traction all the while permitting the drive axles to rotate at different angular velocities. It should be noted that the generic moniker xe2x80x9climited-slip differentialxe2x80x9d is often loosely applied to a family of such devices that perform the above function of limiting slippage and transferring torque to the wheel with greater traction. Strictly speaking, there are four classes of limited-slip differentials: (1) pure limited-slip differentials which limits wheelspin and transfers torque to the wheel with greater traction; (2) self-locking differentials which lock the drive axles together during acceleration; (3) torquesensing differentials which allocate the available torque according to the traction available; and (4) viscous-coupling differentials which use viscous shearing to limit wheel speed differences. It should be noted that some vehicles employ xe2x80x9ctraction controlxe2x80x9d which is not a differential but a control system that actuates the brakes whenever detectors sense the onset of wheelspin. The traction control unit is often coupled with a spark-interrupter or fuel cut-off prevent the brakes from becoming overloaded during operation of the traction control unit.
Among the pure limited-slip differentials are the cam and pawl differential, the clutch pack differential and the active clutch pack differential. While each type varies slightly in structure, all limited-slip differentials utilize some sort of friction couplings to provide resistance to normal differential action. The friction coupling can be clutch plates, clutch cones, governors, or clutch packs. In broad terms, the limited-slip differential functions as follows: when traveling in a straight line, the friction coupling locks the drive axles to the differential case so that torque is delivered to both axles. When cornering, the differential action (i.e. the rotation of the differential gears) releases the friction coupling on the outer axle allowing the outer wheel to rotate more freely than the inner wheel. Consequently, more torque is transferred to the inner axle and wheel.
The oldest limited-slip is the cam and pawl differential, first used by Ferdinand Porsche in 1934. This differential uses an inner cam splined to one drive axle and an outer cam splined to the other drive axle. The cams are mounted concentrically so as to sandwich a plurality of equally spaced pawls (or xe2x80x9cchickletsxe2x80x9d) within a cage-like structure which is integral and rotatable with the differential case and ring gear. In straight-line driving, the cage rotation causes both the inner and outer cams to rotate simultaneously because the pawls become wedged between the inner and outer cams. During cornering, the pawls slip between the cams and thereby allow the differential to function as an open diff. The main shortcoming of the cam and pawl diff is that component wear is substantial and that, once the parts become worn, the locking ability is greatly hindered.
The clutch pack differential is similar to the open diff in construction in that it typically comprises a pair of beveled differential pinions meshed with a pair of beveled side gears. The differential pinions are rotatably mounted about a pinion shaft that is fixed to the differential case thereby allowing the differential pinions to both rotate about the shaft axis and revolve about the axis of the drive shaft (with the case and ring gear). In the clutch pack diff, there are two packs of clutch plates located on either side of the side gears. Each clutch pack has clutch plates alternately mounted to the differential case or to the drive axles so that, when the clutch plates are forced together, the friction therebetween causes the case to become frictionally locked to the drive axles. Thus, when the differential case is driven by the drive pinion, both drive axles are forced to turn since they are frictionally coupled to the case (during straight-line driving). As the torque on the case increases, the spreading force increases and thus the loads pushing the clutch plates together increases. It is this frictional coupling of the clutch plates that limits wheel spin or slippage.
The engagement of the clutch plates is achieved by redirecting the torque exerted on the pinion shaft in a perpendicular (axial) direction. The pinion shaft is mounted at each end between a split thrust member (having angled ramps) which responds to the torque on the pinion shaft by spreading apart and thereby exerting an outward force on the clutch plates. In other words, the angled ramps convert the tangential force (i.e. the quotient of the pinion shaft torque divided by its lever arm) into an axial force proportional to the angle of the ramps. The angle of the ramps can be varied to achieve different performance characteristics. The angle of the ramps on the thrust side (or drive side, i.e. the side against which the pinion shaft exerts its load during forward acceleration) normally has a different angle from that of the coast side (the side against which the pinion shaft presses during engine braking). For the thrust side, a 30-degree ramp angle transmits maximal loads to the clutch plates whereas a 90-degree ramp transmits the least. A typical ramps angle of 45 degrees on the thrust side produces favorable lockup. To produce a minimal locking force during braking, the coast side ramps typically have an angle of 80 or 85 degrees. In addition to the ramp angles, the driving torque, the preload, the stacking of the clutch plates and the material used for the plates are important factors that can be varied so as to produce the desired amount of lockup for limiting slip.
Whereas during straight-line driving, the limited-slip diff frictionally couples both drive axles to the revolving case, during cornering, the limited-slip allows the outer wheel to rotate at a greater angular velocity than the inner wheel. When turning, the differential pinions rotate about their shaft axis and xe2x80x9cwalkxe2x80x9d (revolve) around the slower inner wheel""s side gear. The rotation of the differential pinions releases the pressure exerted by the thrust members on the outer clutch plates. As the pressure is released, the outer drive axle and outer wheel are permitted to rotate at a greater angular velocity. In that situation, most of the torque is being supplied to the inner wheel since it remains firmly clutched to the case.
The active clutch pack differential uses hydraulic pressure to clamp the clutch plates together. The hydraulic actuation of the clutch plates can be regulated by the driver or by a microprocessor so that the diff functions either as an open diff or as a xe2x80x9cspoolxe2x80x9d or closed diff. GKN Axles produces an electrically driven active clutch pack diff that collects data from the car""s ABS system and uses that data to calculate differential wheel speed. The electric motor, through a ball and ramp mechanism, spreads the ramps to clamp the clutch plates whenever the wheel speed differential is excessive.
Self-locking differentials, such as the famous Detroit Locker(trademark), lock the drive axles together during acceleration in straight-line driving. When the car turns, the self-locking diff senses that the angular velocity of the outer wheel is exceeding that of the inner wheel and reacts by unlocking the diff. Thus, the Detroit Locker(trademark) alternates from locked two-wheel drive to unlocked one-wheel drive when turning.
Torque-sensing differentials allocate the available torque according to the traction available. Whereas limited-slips react to the wheel speed differential, torque-sensing diffs deliver torque to the wheel with greater traction before wheel slippage has begun to occur. The Weisman Locker(trademark), used extensively in racecars in the 1960""s and 1970""s, has two roller clutches each having an inner race and a plurality of equally spaced cylindrical rollers mounted in a cage. Each inner race is splined to a drive axle. The outer cam is a single unit that revolves with the ring gear and which houses the inner races and rollers. The rollers are caged between the inner races and the outer cam. Each cage is connected to the inner cams by drag springs.
During straight-line driving, the rollers are wedged between the inner races and the outer cam so that rotation of the ring gear (and outer cam) causes the inner races (and drive axles) to rotate. During engine braking, the diff is locked against the coast side ramps of the outer cam, thereby resulting in two-wheel braking. During cornering, the Weismann Locker(trademark) acts like an open diff. When the angular velocity of the outer wheel attempts to exceed that of the inner wheel, the outer drag spring slips on the inner race, liberating the rollers which then slide down the ramps of the outer cam. Thus, all of the driving torque is transferred to the inner axle and the outer axle is free to rotate at a greater angular velocity through the turn. When torque is reapplied (in a straight-line condition), the outer clutch rollers ramp up again and lock the outer wheel in its normally-closed position. The xe2x80x9ctorque-sensingxe2x80x9d ability arises because, as one wheel begins to slip, the rollers are forced up the ramps of the outer cam causing the diff to lock before slippage has begun to occur.
The Torsen(trademark) differential, manufactured by Zexel-Gleason, is a second example of a torque-sensing diff. The Torsen(trademark) uses three pairs of worm gears meshed with a pair of perpendicularly mounted worm gears that are each splined to a drive axle. The principle of operation relies on the fact that worms transmit torque in essentially only one direction. When one wheel begins to rotate excessively more than its counterpart, the worm gears impede the differential rotation, thus redistributing torque according to traction. In other words, a speed differential between the drive wheels is resisted by the worms which produce a great deal of friction when operated transversely (i.e. in the differential action). Thus, the resistance to differential action causes the torque to be transferred from the wheel with lesser traction to the wheel with greater traction. Since this occurs before the onset of wheelspin, this type of diff is also categorized as xe2x80x9ctorque sensingxe2x80x9d.
The viscous-coupling differential uses the shear-resistance of highly viscous fluids such as silicone gel to couple two substantially parallel clutch plates. Each of the clutch plates are splined to a drive axle whereby when the angular velocity of one of the drive axles begins to exceed the angular velocity of its counterpart, the viscous shearing resistance increases. As shearing resistance is proportional to the speed differential between the left and right drive axles, the viscous diff has a natural propensity to limit slippage between the drive axles. Protracted use of the viscous diff causes heat buildup which eventually leads to the clutch plates contacting one another. This provides maximum lockup for high-torque driving. Twin viscous couplings can also be used in viscous diffs. A twin viscous coupling employs three parallel clutch plates separating two volumes of viscous fluids. Overspeeding of one of the outer clutch plates (which are splined to the drive axles) is resisted by the shearing action in both volumes of viscous fluid.
Some interesting examples of viscous couplings for transferring torque to drive axles are described in U.S. Pat. Nos. 5,881,849, 5,690,201 and 5,632,185 (Theodor Gassman). In the Gassman device, each drive axle is coupled to a pump which produces an actuating pressure proportional to the rotational speed of the axle. The actuating pressure powers a piston in a cylinder which clamps clutch plates together. The clutch plates are alternately mounted to the drive axles and the case. Turning of the case thus causes synchronous rotation of the drive axles.
In light of the foregoing exposition, it should be apparent that there is a fundamental tradeoff between steering and traction in designing a drive train for a four-wheel drive ATV. With a fully open diff, steering is optimized at the expense of traction (no understeer but risk of wheel slippage) whereas with fully locked drive axles, traction is optimized at the expense of steering (understeer but no wheel slippage). The standard prior art solution to this conundrum is to use a regressive limited-slip differential which is essentially a compromise between an open diff and a closed (or locked) diff. The regressive limited-slip differential limits wheel slip and creates only mild understeer. However, like all compromises, the regressive limited-slip does not provide an ideal solution for either steering or traction. Not only is mild understeer bothersome on an ATV that has no power steering, but the torque transfer capacity plateaus in extreme conditions. Thus, there is a need in the ATV industry for a limited-slip capable of functioning as an open diff under certain conditions and then capable of limiting wheel slip by progressively increasing the amount of torque transferred to the wheel with greater traction.
It is thus an object of the present invention to provide an straddle-type all-terrain vehicle with improved traction.
It is another object of the present invention to provide an straddle-type all-terrain vehicle that has improved traction while minimizing the deleterious effects of understeer.
It is another object of the present invention to provide an straddle-type all-terrain vehicle wherein the amount of torque transferred to the wheel with greater traction increases progressively with the differential wheel speed between the wheel with greater traction and the wheel with lesser traction.
It is another object of the present invention to provide an straddle-type all-terrain vehicle having a limited-slip differential capable of functioning as an open differential under certain conditions and then capable of limiting wheel slip by progressively increasing the amount of torque transferred to the wheel with greater traction.
As embodied and broadly described herein, the invention provides a straddle-type all-terrain vehicle including a chassis and an engine mounted to said chassis; said engine including a crankshaft geared to an output shaft; said output shaft being connected to a drive shaft; said drive shaft being connected to a limited-slip differential; said limited-slip differential, in operation, capable of distributing torque to a left axle coupled to a left wheel and to a right axle coupled to a right wheel; said differential adapted to permit said left and right axles to rotate at different angular velocities and further adapted to limit slip by progressively redistributing torque to the wheel with greater traction when a speed differential between said axles exceeds a predetermined threshold.
The straddle-type all-terrain vehicle of the present invention has a differential that functions initially as an open differential in that it allows the vehicle""s left and right axles to rotate at different angular velocities. However, when one of the wheels slips, the speed differential exceeds the threshold, termed the xe2x80x98cutting pointxe2x80x99, and the differential then begins to function as a limited-slip differential, limiting slip by transferring torque to the wheel with greater traction in proportion to the wheel speed difference. Whereas prior art ATV differentials are regressive in that their ability to transfer torque decreases as the wheel speed differential increases (eventually reaching a plateau), the progressive limited-slip differential can transfer more and more torque to the wheel with greater traction (does not reach a plateau). It should be noted that while this particular embodiment of an ATV with a progressive differential does not reach a plateau, by reducing the number of clutch plates (from 7 inner plates and 8 outer plates to, say, 5 inner and 6 outer plates) or by varying other attributes of the progressive limited-slip differential, the torque transfer can be made to reach a plateau for a certain wheel speed differential.
Thus, an ATV with a progressive differential provides fundamental advantages over the prior art in steering and traction. Since the progressive limited-slip does not engage until the cutting point, the steering of the ATV (below the cutting point) is unhindered by the effects of understeer (a phenomenon that is particularly detrimental to ATV""s since they do not have power steering) and thus require a smaller steering effort from the operator. In conditions of extremely poor traction, when wheel speed differentials become large, an ATV with a progressive differential can transfer relatively more torque to the wheel with greater traction than the prior art regressive limited-slip differential.
Preferably, the front differential comprises a drive pinion meshed with a ring gear, said ring gear being fixed to a differential case, said differential case having a pair of differential pinions meshed to a pair of side gears, said side gears being coupled to said axles, a rotary pump responsive to wheel speed differential, said pump capable of generating a hydraulic pressure in a piston for exerting an axial force on a clutch pack for frictionally coupling said axles, thereby transferring torque from the axle with a higher angular velocity to the axle with a lower angular velocity.
This arrangement ensures that the force exerted on the clutch pack by the piston is directly proportional to the angular velocity of the pump and that the pump""s angular velocity is directly related to the wheel speed differential. Thus, the torque transfer is almost directly proportional to wheel speed differential (hence the moniker xe2x80x9cprogressivexe2x80x9d limited-slip differential).
Preferably, the differential case further comprises a cross shaft traversing said differential case and anchored therein for rotation therewith, said differential pinions being rotatably mounted on said cross shaft whereby said differential pinions are able to revolve with the differential case and ring gear and are also able to rotate about the axis of the cross shaft to compensate for the speed differential between said axles.
Below the cutting point, the differential pinions and side gears permit a xe2x80x9cdifferential actionxe2x80x9d when cornering (i.e. functioning like the standard open differential). When operating slightly above the cutting point, even though the clutch packs are frictionally coupling the two axles together, the coupling force may still be weak enough to permit a partial differential action, meaning that there is some degree of slip between the clutch pack and the side gears, allowing the differential pinions to rotate somewhat.
Preferably, the predetermined threshold of speed differential is less than 10 revolutions per minute and, more preferably, ranges from 1 to 3 revolutions per minute.
The cutting point can be varied to either reduce steering effort or increase traction. By raising the cutting point, the progressive limited-slip differential functions as an open diff for a longer period of time before the clutch pack is frictionally engaged. This favors steering since the wheel speed differential during cornering does not cause the clutch packs to engage. However, when one wheel begins to slip, there is a longer lag until the differential begins to transfer torque to the wheel with greater traction. By lowering the cutting point, the differential is frictionally engaged more readily, even perhaps during sharp cornering (which could lead to understeer). However, at the onset of wheel slip the clutch packs begin to frictionally engage more readily, giving better traction response during slippery conditions.
It should be noted that the cutting point is specific to the vehicle. It is calibrated so as to provide a compromise between steering effort and traction. The calibration of the cutting point is a function of vehicle dynamics (e.g. steering geometry, wheelbase, wheeltrack, weight distribution, etc.). Thus, it should be appreciated that a cutting point will vary from vehicle to vehicle.
Preferably, when the speed differential is less than the threshold, there is a small, constant torque transfer between the axles resulting from pre-load in the limited-slip differential.
A small measure of frictional coupling is desirable for situations when the wheels are both spinning simultaneously. This is achieved by means of a small spring (or equivalent) that exerts a minimal axial load against the clutch packs.
Preferably, the ATV also has a rear output shaft geared to said crankshaft, a rear drive shaft connected to said rear output shaft, a rear differential connected to said rear drive shaft, said rear differential having a drive pinion and ring gear for distributing torque to rear axles, whereby said straddle-type all-terrain vehicle operates as a four-wheel drive.
Although the cornering dynamics of an ATV are such that a rear closed differential is adequate for light rear loads, it becomes necessary for heavy rear loads to employ a rear limited-slip differential (preferably a progressive diff) to provide optimal steering and traction.
Other objects and features of the invention will become apparent by reference to the following description and the drawings.